1. Field of the Invention
The present invention relates to an optical pickup device providing a high performance at a low cost and a recording medium used therefor.
2. Description of the Related Art
In order to increase information recording density with respect to a recording medium, the size of a light spot irradiated on a recording medium for recording and/or reproduction needs to be reduced. Since the size of a light spot is proportional to the wavelength λ of a light beam used and inversely proportional to the numerical aperture (NA) of an objective lens, as shown below in Formula 1, the recording density increases as the NA of the objective lens increases:Size of light spot ∝λ/NA  Formula 1
However, as is generally known, an objective lens formed of a single lens is difficult to manufacture to have a high NA, for example, 0.7 or more, while satisfying allowable conditions of optical aberration (decenter>4 μm and tilt>2′ when a slope angle is 55 degrees and an OPD=0.03λ rms) due to manufacturing limitations. This is because the material for the lens is limited and the shape of a spherical surface of the lens is not designed to be suitable for injection. That is, in an objective lens formed of a single lens having a short focal length and a high NA, the curvature of a spherical surface thereof increases and an angle of a tangent line at an effective surface becomes over 55 degrees. This slope angle makes manufacture of a mold difficult and decreases an allowable decenter between lens surfaces when a lens is manufactured, thus dropping production yield. Thus, an objective lens which is easy to manufacture and achieves an assembly allowance while having a high NA is needed.
Also, since an effect of wavefront aberration according to a degree of the thickness of a recording medium is sensitive to an increase as an NA increases, the thickness of a recording medium (i.e., the thickness of a protective layer for protecting a recording surface) needs to be thin.
Meanwhile, as is widely known in the field of technology to which the present invention pertains, a DVD (digital versatile disc) has a 0.6 mm thick protective layer to protect a recording surface of a recording medium. When the protective layer is thick as in the DVD, a sufficient working distance (WD) between an objective lens and a recording surface of a recording medium is needed. The thickness of the protective layer serves as a limit in increasing of the NA of the objective lens to minimize the objective lens formed of a single lens and high density recording. This is because, when an objective lens having a working distance sufficient to prevent contact with a recording medium and to enable a small diameter is manufactured, the curvature of a spherical surface increases and the amount of allowable decenter between lens surfaces and the amount of an allowable angle of view decreases and productivity deteriorates.
To satisfy the requirements for the thickness of a thin protective layer and overcome the limit of the objective lens formed of a single lens, an objective lens 10 formed of two lenses, such as the lenses disclosed in Japanese Patent Publication No. 10-123410, has been developed as shown in FIG. 1. Referring to FIG. 1, the conventional objective lens 10 includes a first lens 11 to condense an incident light beam, and a second lens 13 disposed between the first lens 11 and a recording medium 1 to increase the NA of the objective lens 10. When a light beam having a red wavelength, for example, 635 nm or 650 nm is used, to secure assembly allowance, a curvature is assigned to four lens surfaces so that a high NA of 0.7 or more can be obtained.
The objective lens 10 has at least one aspheric surface and a low dispersion material is used to reduce chromatism. The NA of 0.7 or more can be obtained by using a low dispersion material having an Abbe's number of 40 or more in the helium line d (wavelength of 587.5618). Also, an NA of 0.8 or more can be obtained by using a low dispersion material having an Abbe's number of 60 or more in the helium line d.
Assuming that the focal length of the first lens 11 is f1 and the synthesized focal length of the two lenses 11 and 13 (i.e., the total focal length of the objective lens 10) is f, the objective lens 10 satisfies the inequality of 1.7<f1/f<2.5.
Also, when the thickness T′ of a transparent protective layer used to protect the recording surface of the recording medium 1 with respect to the NA satisfies conditions of the following Formula 2, correcting aberration of the objective lens 10 is possible:When 0.7≦NA<0.8, T′≦0.32 mm;When 0.8≦NA<0.9, T′≦0.20 mm; andWhen 0.9≦NA, T′≦0.11 mm.  Formula 2
If the objective lens 10 satisfies the condition that n1>n2, where n1 is the refractive index of a lens having a relatively greater curvature of the two lenses 11 and 13 and n2 is the refractive index of a lens having a relatively smaller curvature, and assuming that BW is the diameter of an incident light beam and WD′ is a working distance, the objective lens 10 satisfies the condition that 1.0≦BW<4.5 and 0.05≦WD′. The diameter BW of the light beam and the working distance WD′ satisfy the conditions of Formula 3 shown below with respect to the NA of the objective lens 10:When 0.7≦NA<0.8, WD′≦0.25676BW+0.039189;When 0.8≦NA<0.9, WD′≦0.14054BW−0.064865; andWhen 0.9≦NA, WD′≦0.096429BW−0.244640.  Formula 3
The conventional objective lens 10 can realize a high NA of 0.8 or more with respect to a recording medium having a protective layer of which the thickness is thinner than that of a DVD. Also, the objective lens 10 formed of two lenses 11 and 13 satisfies the requirement of a thin protective layer and can overcome the limit of an objective lens formed of a single lens. However, the conventional objective lens 10 formed of two lenses needs a holder to maintain the distance between the lenses 11 and 13 (i.e., an air gap). Thus, since the conventional objective lens 10 has a high NA, a distance allowance and inclination allowance between the lenses 11 and 13 are very strictly obeyed and the production yield deteriorates. Such a deterioration in the production yield causes an increase in the manufacturing cost of an optical pickup device using the objective lens 10. Also, when the conventional objective lens 10 is used, since an actuator is needed to perform focus control and so forth by integrally driving the two lenses 11 and 13, a recording/reproducing apparatus is made complicated and huge.
Japanese Patent Publication No. 10-123410 discloses an example in which the conventional objective lens 10 is designed with respect to a wavelength of 635 nm and 650 nm. This signifies that an optical pickup device employing the conventional objective lens 10 uses a light source for a 635 nm or 650 nm wavelength.
However, since the unit price of the light source for a 635 nm or 650 nm wavelength is very high as compared to a light source for a relatively longer wavelength (for example, a light source used for a recording medium of a CD family), it is difficult to lower the price of an optical pickup device.